This type represents a DOM element's attribute as an object. In most DOM methods, you will probably directly retrieve the attribute as a string (e.g., Element.getAttribute(), but certain functions (e.g., Element.getAttributeNode()) or means of iterating give Attr types.

Warning: Starting in Gecko 7.0 (Firefox 7.0 / Thunderbird 7.0 / SeaMonkey 2.4), the ones that are going to be removed output warning messages to the console. You should revise your code accordingly. See Deprecated properties and methods for a complete list.

Note: DOM Level 4 removed this property. The assumption was that since you get an Attr object from an Element, you should already know the associated element.
As that doesn't hold true in cases like Attr objects being returned by Document.evaluate, the DOM Living Standard reintroduced the property.

This property always returns true. Originally, it returned true if the attribute was explicitly specified in the source code or by a script, and false if its value came from the default one defined in the document's DTD.

Indicates whether the attribute is an "ID attribute". An "ID attribute" being an attribute which value is expected to be unique across a DOM Document. In HTML DOM, "id" is the only ID attribute, but XML documents could define others. Whether or not an attribute is unique is often determined by a DTD or other schema description.

You shouldn't have been using this in the first place, so you probably don't care that this is going away.

parentNode

This property now always returns NULL.

previousSibling

This property now always returns NULL.

schemaTypeInfoRead only

The type information associated with this attribute. While the type information contained in this attribute is guaranteed to be correct after loading the document or invoking Document.normalizeDocument, this property may not be reliable if the node was moved.